Typically such systems provide that the rear wheels steer in the same direction as the front wheels in modes of operation associated with high vehicle speed, providing a sideshifting characteristic to the vehicle so that the same level of lateral acceleration response is achieved with a lesser degree of yawing. The resulting enhanced lateral response of the vehicle to steering wheel inputs gives the driver a sensation of stiffness and security, particularly in lane changing at high speeds. On the other hand some such systems provide that the rear wheels steer in opposite direction to the front wheels in modes of operation associated with low vehicle speed, providing an enhanced yaw response with consequential good maneuverability and minimum width of the "swept path" of the vehicle through corners. In this specification "swept path" is defined as the envelope of the vehicle's trajectory projected with respect to the road during a maneuver. The numerical ratio of rear steer angle to front steer angle, measured instantaneously over a small incremental range of these variables at a given vehicle speed, will be termed "rear/front steering ratio". This ratio is therefore the effective instantaneous slope of the rear steer angle function when plotted against front steer angle at a given vehicle speed. According to this definition, rear/front steering ratio is usually designed to be positive at high vehicle speeds and negative at low vehicle speeds. Also the term "on-center" will be used to describe the driving condition when the steering wheel is positioned in or around its central operating region, as associated with straight-ahead driving.
The optimum rear/front steering ratio for high-speed on-center driving has been found-to be approximately 0.4. Ratios higher than 0.4, although further improving lane holding and lane changing during highway driving, lead to inadequate vehicle yaw response during negotiation of highway curves. Similarly the optimum rear/front steering ratio for low speed parking maneuvers has been found to be approximately -0.3. Values lower than -0.3 (for example -0.5 or -1.0), although providing excellent vehicle maneuverability, may cause the rear of the vehicle to "sweep-out" and rub against the curb in parking maneuvers. Non-experienced drivers find excessively low values of rear/front steering ratio difficult to master.
The additional major constraint on virtually all 4WS systems is that rear steer angles must be limited to about 5 degrees in order to avoid the rear wheels intruding into potential rear trunk compartment space. Although 5 degrees rear steer angle represents a practical limit in low speed maneuvering consistent with the aforementioned sweep-out problem during parking, 5 degrees rear steer angle will be extremely dangerous in high speed driving. In some prior art speed dependent 4WS systems, 5 degrees rear steer will inevitably occur for large magnitudes of steering wheel input because of the particular speed dependent reversing mechanisms employed. The design philosophy in this case is simply based on the fact that such large steer wheel angles will never be used in normal high speed driving. This is of course correct, however, in emergency evasive maneuvers at high speed, or recovery from skidding, large steering angles will often be necessary. In such critical driving situations it is essential that large rear steer angles do not occur in order that the response of the 4WS vehicle is not disconcerting to the average driver.
Mechanical sliding mechanisms such as disclosed in U.S. Pat. No. 4,313,514 (Honda), U.S. Pat. No. 4,552,239 (Mazda), and west German Patent 3,837,141 (ZF) maintain a constant rear/front steering ratio independent of front steer angle, for a given vehicle speed. In order to limit rear steer to the abovementioned 5 degrees for a typical maximum front steer angle of 36 degrees, such mechanisms must limit the rear/front steering ratio to about 0.14 (5/30). This, however, is still only 35% of the previously mentioned optimum on-center value of 0.4 at high speed and 47% of the allowable value of 0.3 at low speed and parking.
For this reason more sophisticated mechanisms have been developed which enable rear/front steering ratio to not only be varied as a function of vehicle speed but also, for any given speed, varied as a function of front steer angle. These mechanisms enable a higher rear/front steering ratio to be used on-center yet, by reducing this ratio towards full-lock, maintain the constraints previously described in respect to this ratio overall, hence limiting maximum rear steer angle. By way of example the mechanism disclosed in U.S. Pat. No. 4,572,316 (Mazda), and as used in the production of 1989 Mazda MX6 vehicle, incorporates a disk rotatably supported on a nutating axis, the inclination of this axis being actuated by stepper motor as a function of vehicle speed. The disk is inclinable either side of a central null position and drives a spool valve supported co-axially with its axis at this null position via a ball-jointed push rod connected to its outer periphery. Displacement of the spool valve activates the hydraulically assisted rear steer, providing the required positive rear/front steering ratio at high speed and negative ratio at low speed. A front steer angle causes the disk to rotate on its inclined axis giving a sinusoidal variation in rear/front steering ratio with a magnitude approximately proportional to the angle of inclination of the axis of the disk with respect to its null position. At the null position therefore zero rear steer occurs independent of any given front steer angle. The vehicle speed for which the resulting rear/front steering ratio is zero will be termed the "crossover speed" and is typically set at moderate speeds of about 30-50 km/h.
The above described sinusoidal variation results, for any given vehicle speed, in the rear/front steering ratio on-center being 1.57 times its mean (overall) value. Hence, for the data exampled earlier, during high speed driving an on-center rear/front steering ratio of 0.22 (1.57.times.5/36) could be provided while still limiting rear steer at maximum steering wheel input to 5 degrees. This value is still only 55% of the optimum value of 0.4.
Allowed U.S. Pat. application No. 07/849,369 (Bishop) overcomes this limitation of the nutating mechanism described in U.S. Pat. No. 4,572,316. In one embodiment of the invention disclosed in this Patent Application, a front-steer angle dependent function generating mechanism is shown which is in the form of a pin and slot device similar to one quadrant of a Geneva mechanism wherein the driving member rotates about an axis and carried a pin offset from that axis which engages a pivoted driven member having a radial slot. The ratio between the offset of the pin in the driving member and the radius at which the pin engages the slot of the driven member, with respect to its own axis, determines the basic characteristic of the mechanism. Such a device can provide a relationship between input and output which is non-sinusoidal having a maximum ratio say 3 times that of the mean overall ratio. In terms of the previously quoted example, the high speed on-center rear/front steering ratio can be raised to 0.42 (3.times.5/36) using this mechanism. Thus the optimum value of 0.40 can be provided. However, as in the case of the nutating mechanism shown in U.S. Pat. No. 4,572,316, large on-center rear/front steering ratios are only achieved at the expense of permitting unacceptably high rear steer angles (approaching 5 degrees) for high speed maneuvers requiring large steering wheel inputs. This represents a first disadvantage of such prior art mechanisms.
A swinging link mechanism is further described in U.S. Pat. application No. 07/849,369, this mechanism essentially being arranged in series with the abovementioned Geneva type front steer angle dependent function generating mechanism. Therefore, as in U.S. Pat. No. 4,572,316, the speed dependent function of the overall mechanism "scales" the magnitude of the front steer angle function which is basically a fixed relationship based on the geometry of the mechanism. Again a crossover speed exists for which the rear/front steering ratio is zero for all values of front steer angle.
The existence of a crossover speed for the function generating mechanisms disclosed in U.S. Pat. No. 4,572,316 and U.S. Pat. application No. 07/849,369 means that, in the region of this crossover speed (say 30-50 km/h) the performance of a 4WS system is little different to a traditional two wheel steering system. This is simply because only very small, if any, rear wheel steer angle is generated for a given front wheel steer angle for speeds in this range and represents a second disadvantage of such prior art mechanisms. Various attempts have been made to overcome this basic shortcoming. Mazda usually programs a rapid change in the speed dependent relationship through the crossover speed. In other words, the magnitude of the sinusoidal variation in rear/front steering ratio is quickly increased from a large negative value, through zero, to a large positive value as the vehicle accelerates through the crossover speed. Such a relationship for the Mazda MX6 vehicle has been disclosed in the publication SAE-Australasia Journal July/August 1989 and also in the Mazda technical publication "4WS - The Mazda Speed Sensing Computerized 4-Wheel Steering System Technical Information", Mazda Australia Pry Limited 1989. Such a very non-linear speed dependent relationship however leads to a corresponding very non-linear variation in vehicle yaw rate response through the crossover speed. Normally around these moderate speeds, a smooth and progressive increase in vehicle understeer would be expected by the average driver as vehicle speed increases.
Another technique proposed for making the crossover speed region of such systems less obvious to the driver is to purposely introduce hysteresis to the speed dependent relationship. This results in a differing crossover speed depending on whether a vehicle accelerates or decelerates through the crossover speed, or depending on different road conditions. Such concepts are disclosed in U.S. Pat. No. 4,730,839 (Mazda) and U.S. Pat. No. 4,733,878 (Honda). Numerous examples exist of concepts involving manually adjustable crossover speeds, purportedly to allow the driver to adjust the vehicle dynamics to suit his/her own subjective likes and dislikes, for example, U.S. Pat. No. 4,660,844 (Honda) and GB Patent 2,173,460 (Honda).
The above techniques however do not overcome the inherent disadvantage in prior art 4WS system design in which, at a certain crossover speed, zero rear steer angle occurs irrespective of front steer angle. Now it is well known in the art of automotive vehicle dynamics that, for a typical vehicle whose center of mass is biased toward the front of the vehicle, the vehicle yaw rate response as a function of steering wheel angle input decreases for increase in speed. This increasing understeer characteristic at increased speed for a given steering wheel input is typical for almost all commercially available cars and has a very deleterious effect on vehicle dynamics above about 0.3 g lateral acceleration due to the interaction between suspension design and tire characteristics. This understeering characteristic can lead inexperienced drivers to "run wide" in a turn or an emergency maneuver. Unlike in a two wheel steered vehicle, in a 4WS vehicle it is possible to counteract this characteristic at medium and high vehicle speeds by steering the rear wheels to generate a neutralizing (or partially neutralizing) oversteering trend at higher lateral accelerations.
In order to accomplish this in the region of crossover speeds (say 30-50 km/h) it is desirable that, even though zero rear wheel steer occurs for small steering wheel angles either side of on-center, for larger steering wheel angles associated with lateral accelerations of 0.3 g or more the rear wheels steer in the opposite direction to the front wheels. This progressive increase in negative rear/front steering ratio for higher lateral acceleration levels, up to some predetermined limit, will conveniently tend to counteract the undesirable progressively increasing understeer characteristic inherent in conventional vehicle dynamics due to the aforementioned suspension and tire characteristics. Taken a step further, the vehicle can even be made to exhibit a moderate oversteer characteristic as, for example, used by normal drivers in severe cornering or by experienced drivers when maneuvering through a marked slalom course. This handling characteristic will result in a much reduced width of swept path of the vehicle and consequently superior avoidance maneuver and slalom course performance.
A similar philosophy can be applied at vehicle speeds above the crossover speed where, in accordance with the above referred to prior art speed dependent 4WS systems, the rear/front steering ratio is positive for all front steer angles so reducing the vehicle yaw response. Thus the magnitude of this positive rear/front steering ratio could be decreased for higher lateral acceleration levels, as occurs in high speed cornering, in order to restore the yaw response of the vehicle to steering wheel inputs. For certain vehicles it may be advantageous to limit or even reduce rear steer angle, as a function of front steer angle, above a certain threshold front steer angle level. Limiting of rear steer angle has already been referred to as having advantages not only in terms of vehicle dynamics, as perceived by the driver, but also safety. U.S. Pat. No. 4,552,239 (Mazda) discloses this philosophy for vehicle speeds well above the crossover speed, that is for speeds at which the on-center rear/front steering ratio is much greater than zero. No disclosure is made in that patent regarding the rear steer philosophy that should be adopted for speeds in the region of the crossover speed or below this speed. Two embodiments are described in that patent: the first is a microprocessor driven electro-hydraulic system which is effectively "fly-by-wire" and, the second, the earlier referred to mechanical sliding mechanism. The first embodiment described may possibly be able to adhere to the rear steer philosophy disclosed but little teaching exists to this effect. However, as elaborated on later, such "fly-by-wire" systems are unlikely to be adopted in mass production vehicles in the near future for reasons of safety. The second embodiment described appears somewhat "out of place" since that relatively simple sliding mechanism cannot possibly adhere to the complex rear steer philosophy disclosed, in fact it would only be able to produce a constant rear/front steering ratio. Thus that mechanism inherently must possess a crossover speed for which zero rear steer occurs independent of front steer angle.
As stated earlier, the mechanisms disclosed in U.S. Pat. No. 4,572,316 or U.S. Pat. application No. 07/849,369 have the speed function generating mechanism arranged in series with the front steer angle function generating mechanism. Such compound mechanisms therefore necessarily produce a family of front steer/rear steer relationships which are "scaled replicas" as a function of vehicle speed. This third disadvantage of such prior art mechanisms is compounded by the fact that, at the crossover speed, this scale factor is necessarily zero in order to yield a zero rear/front steering ratio on-center. However this, by default, yields zero rear steer angle for any front steer angle at the crossover speed which is clearly, as stated above, a suboptimal situation in terms of vehicle dynamic requirements. A fourth related disadvantage of such prior art mechanisms is that, for any given vehicle speed, the relationship between rear steer angle output and front steer angle input is a smooth, relatively simplistic, function. As seen, the nutating mechanism disclosed in U.S. Pat. No. 4,572,316 generates, basically, a sinusoidal relationship. The Geneva mechanism disclosed in U.S. Pat. application No. 07/849,369 offers a higher on-center rear/front steering ratio, but nevertheless is incapable of generating the earlier referred to compensation for the understeering characteristic of the vehicle.
It is, of course, possible to design a fully electrically actuated function generating system which would enable microprocessor control of numerous complex rear steer control strategies as a function of front steer angle and vehicle speed- Such systems are disclosed for example in U.S. Pat. No. 4,418,780 (Nissan), U.S. Pat. No. 4,552,239 (Mazda), U.S. Pat. No. 4,733,878 (Mazda) and GB Patent 2,173,460 (Honda), however their lack of commercial adoption most likely stems from the inherent dangers associated with having the rear steer Control effectively "fly-by-wire". Mechanically actuated hydraulic steering systems have an historically proven safety record in the area of conventional power assisted front steering of cars. It is commercially acceptable to use electrical actuation to provide the speed input to a speed dependent 4WS system, as shown in U.S. Pat. No. 4,572,316 and used in the Mazda MX6 vehicle, since even if through malfunctioning this input goes to maximum or zero, the vehicle still remains steerable.
In light of the current impracticality of electronically actuated function generating systems with regard to safety, and based on the above described disadvantages of prior art mechanically actuated function generating mechanisms, a mechanical function generating mechanism is proposed for which the front steer angle and vehicle speed, the two input variables, interactively combine to "calculate" the required rear steer angle, the output variable. Stated mathematically, these variables are not "independent" as far as the determination of rear steer angle is concerned. The relationship between front steer angle and rear steer angle at, say, a low speed of 10 km/h will not necessarily even resemble the general form of the relationship near the crossover speed of say 45 km/h, or indeed at high speeds of say 200 km/h. Such optimised interaction of these variables should ideally be available at all vehicles speeds. The optimum characteristic relating front and rear steer angle variables for any given vehicle speed, being derived from non-linear tire and suspension characteristics, will never be a straight-fine and will almost necessarily require re-entrancies. Thus the straight-line characteristics lacking re-entrancies, illustrated for example in U.S. Pat. No. 4,552,239 (Mazda), are suboptimal.